Long time, no post, I know......I've been otherwise.......occupied. Sorry

I have a small favor to ask of you all. I recently went to reassemble my Electra stompbox board, and realized my 504CH chorus, while sounding great on battery, makes an unholy hum when powered with a DC supply. The polarization is correct. It's fine when it's off, but when the effect is activated.....hum city.

After popping her open, it's apparent that someone has been fiddling about in here, particularly at the DC supply plug - surprise, surprise.

So, this brings up my favor. Does anyone have one of these that could snap a pic or two and see if it's just wired wrong, or perhaps point me in the right direction? Schematics are non existent from what I can see.....

I've attached a pic of the pedal itself, and a pic of the guts for comparison. I've tried resizing them several times. Not sure why they are so huge. If you open them in a new tab or right click and "View Image", they are much more managable. My apologies.....

is your power device grounded? If so, try a ground lift adapter. I had that happen before.

Next - if it is not grounded, can you flip the direction it is plugged in?

Does your adapter hum with other pedals?

The other solution may be a new supply. If you are getting one make sure the plug and polarity are correct. I'd suggest trying one out in a Guitar Center if you can (or some other local store) - they will let you bring your pedal in to try it. That way you KNOW it will work. You might pay a few bucks more than online but you won't be buying something that won't solve the problem and have to ship it back.

Thorny! I had forgotten about you being our resident electronics guru, sorry about that! Thanks for chiming in!

In answer to your questions, power source is a cheapo radio shack multi-voltage supply. It is not grounded, and is not reversible as to which direction it is plugged in due to one prong being larger. I did however, use some jumpers to get the hot and neutral reversed on the power supply, and it didn't seem to make a difference in how I connected it.

This same adapter does not seem to hum with other pedals, but it does make my Sioux Guitars tremolo pedal hiss and sound a little goofy, so I think next step is, as you suggest, go find a good clean supply for this pedal.

Yes I've seen similar situations. The power supply isn't regulated and filtered well enough. Looking at the photo there's nothing there that looks out of the ordinary. I have a couple pedals from the 500 series and they look about the same inside. By the way, I have a friend who holds this mono version of the 500 chorus as *almost* his favorite chorus pedal ever. He says it's a little too noisy, but the tone is great. And I don't mean the power supply noise you're fighting here-- but rather the general hiss/background noise from the analog circuit.

Thanks ultra sonic. I agree with your friend. On battery, this is the only chorus in my chain for certain amps. If you need squeaky clean or super subtle effect, then yes, there are much quieter out there, but I do love the modulation of this era of chorus, phase, flange, etc.

She's on battery for the time being. Can't find a supply quiet enough. Would it help at all to run the supply into an RC circuit before the pedal and drop the voltage a bit along with the added filtering? The cruddy supplies I have lying about all run higher than 9, sometimes nearly 11 volts...??

Hey, I noticed a $25 electra regulated power supply on ebay (at $25 now, who knows what it will sell for). It looks period correct. It might be worth a gamble if it goes real cheap. I have no knowledge of the seller or anything about it. I have not had one of those. It makes sense that if it works, it should not be noisy with its own power supply. Notice there are a lot of "ifs" in that paragraph.

You can try additional filtering if you want to - you mentioned it lowering voltage, as long as result it at least 9V or a little more that should be fine. If you have a scope, you can measure how much ripple is on your power supply before and after. You can get an old scope to test or a new meter-based one is not terribly expensive. But something to borrow not to buy if you'd never use it again, or just wing it. You at least have a multi-meter.

I confess I usually am working on guitar amp power sections not the small stuff. You can try filtering what you have more, you can try the electra power adapter as is, or replace caps in the device (likely accessible). If using a current device can you place additional filtering inside? It is a matter of packaging. You might if it is in a box hammond style box like he electra one was. But most current stuff is a little brick, not something you can open up and get into.

Also, one can check and possibly change electrolytic caps in the pedal, I have had to do that with my phaser pedal of the same age and some of the modules I have had. These parts are meant for a 10 year life span and we get 40+ years out of them, so we got our money's worth. It could be some flaky caps (no longer up to spec) make the pedal more susceptible to the noise. That is just a WAG (wild a$$ guess for those not familiar with that engineering term). The tantalum capacitors (if it has any) are the "tear drop" capacitors - have color codes on them and a little "+" positive sign because they are electrolytics. Don't forget those. If these go out the pedal will quit working, but there is a time when it is in between that the audio results can be less predictable. Amps run into this, and one of the sounds it makes is called "motorboating" because it kind of sounds that way. When that happens they need recapped. But I digress.

Shoot, the $25 for the Electra supply was the buy it now price, so I did

That's a good point about the caps, I do see several electrolytics in there. Just because they're lower voltage doesn't mean they don't suffer the same fate as their big brothers, huh?

I think I have access to an old CRT scope I can borrow. That would definitely help in determining which supply is best to start with and if any additional filtering is doing any good. My main concern with an added filter is sizing the components. The resistor will have to be pretty small. What, if anything, should I then do with the capacitor value? Raise, lower, leave it? I'm also thinking I don't need to worry about impedance, but should I?

Anything electrolytic inside the pedal or inside the electra power adapter might benefit from a replacement with as close to the same type and value as possible. for instance, if it there is a 20mfd at 25v and you have a 22mfd at 50v - you will be fine (same or higher voltage, as near as possible to the MFD value). Don't go lower voltage (not likely a problem here, but just saying). Get as close as you can on the value. Take good pictures before you start working on anything. Make note of exact values and where located when removing and the + and - side or the cap. Any new electrolytic cap should be as good or better than the originals.

I've had pretty good luck with the Voo Doo lab pedal power. Has 2 adjustable out puts to lower the voltage to simulate battery drain for pedals that sound better on lower voltage. Try to catch it on sale if possible.

Would it help at all to run the supply into an RC circuit before the pedal and drop the voltage a bit along with the added filtering? The cruddy supplies I have lying about all run higher than 9, sometimes nearly 11 volts...??

The voltage on cheap supplies is high without a load because they are not very well regulated. They "overshoot" the target voltage because it will load down as more current is drawn by the pedals it's powering. A better design would be sitting right at the target voltage with no load, and adding pedals to it would not cause the voltage to sag. A VooDoo labs supply should be good, as someone mentioned-- you need both good filtering and good regulation. I use a VooDoo in the studio and it works very well.

The old Electra supply might work, but I'd definitely refurbish the electrolytic capacitors. Go with higher voltage options as Thorny pointed out, but with regulated power supply circuits I think you can increase the MFD as well. So if you have a 20mfd @ 25vdc, get a 50vdc at as high a capacity as will physically fit in the space without major trouble, like 47mfd. Also, if you have any choice in the matter, always opt for caps that are rated for a higher temperature. They'll last longer and be more stable. Many are rated to 85 degrees C, but go for ones rated at 105 degrees C if it's not too expensive for your purposes.